Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.

Populations in ecosystems


A number of terms used in ecology have slightly different meanings than they do in other contexts.


A population is a group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed. It is often also used as a quantitative term, covering the number of organisms in that space.

Ecology is the study of organisms in their natural environment. Eco- means 'home'.

The term population also features prominently in regard to genetics and speciation.

Population is often used in the context of humans and the number of individuals involved.


The term habitat covers the type of natural environment or surroundings in which an organism usually lives. It is a noun, but it is derived from the Latin for 'it lives', and where an organism lives is a key factor in describing or defining a new species.
Yellow-footed tortoise

640px-Yellow-footed_tortoise (111K)

This description was used (in 1824?) to describe a specimen taken as a result of an expedition to Brazil: "Habitat in sylvis juxta flumen Amazonum"
It has been called Testudo sculpta and Geochelone denticulata but it is currently known as Chelonoidis denticulatus.


Within a habitat, a species occupies a niche which is a description of its biological role - not a geographical space. A species has adaptations that enable it to perform this role, and it thus reacts successfully to the prevailing abiotic and biotic conditions in this area of the environment.
Arctic Hare in snow (55K)

The Arctic hare Lepus arcticus occupies the niche of herbivore, feeding mainly on Arctic willow within the Arctic tundra, principally in northern Canada.

Its distinctive coat of thick fur is its primary adaptation; it provides insulation against low temperatures which is the principal abiotic factor, and its white colour also camouflages it in the snow, thus protecting it from predators, mainly Arctic wolves and foxes - clearly an important biotic factor.
360px-Uffizi_19,_Farinata_degli_uberti (31K) In architecture a niche is a shallow recess, especially one in a wall to display a statue or other ornament

In everyday human terms the term niche can mean 'a comfortable or suitable position in life or employment' and this is close to its ecological meaning.

It is practically a job description within an ecosystem. It is said that each species has its own niche and it cannot overlap with other species.

The adaptations we are talking about are innate properties of the species, not the result of individual choices or decisions. Of course humans are exceptions; we can make fairly complex day-to-day changes to our lifestyle to fit in with environmental factors.


A community consists of a number of different populations, i.e. different species, which interact within an area.

Species richness is the number of different species in an ecological community, or an area within a landscape. Having established a species list by an initial examination of an area, more information may be gathered about the numbers of each organism.

Species diversity takes into account both species richness and (the evenness of) the abundance of those species.

A community is effectively all the organisms of all the species (or all the populations) which are found within a habitat, ecosystem or area.

The biodiversity index can be used in comparisons between such areas or at different times.


This is calculated from the total number of organisms N of all the species and the total number of organisms of each species n. A value of 1 shows no biodiversity (just a single species), whereas any increase in numbers of species takes the value above 1.


An ecosystem is a combination of living organisms - a community or communities - together with the non-living environment.

It is said that ecosystems can vary in size from very small to very large. In general an ecosytem covers a wider area than the habitat of a single species.

Although it is tempting to see the earth as a single ecosystem, it can clearly be divided into a number of distinct zones, each with their own climate and characteristic populations of living organisms. These zones are called biomes, but each one is capable of being subdivided further.

Land Biomes Water Biomes
Taiga (boreal forest)
Temperate forest
Coral reef

Factors affecting population size

Carrying capacity

This is the (maximum) size of a population that can be maintained within an ecosystem.

A population requires a number of factors (resources): 'food', water, space within the habitat, etc, and their supply needs to be sustained - otherwise the habitat may be degraded.

The population size carried may be affected by a combination of abiotic factors (physical aspects of the environment) and biotic factors (due to interactions with other living organisms) - principally competition (interspecific and intraspecific) and predation (predator/prey relationships).

The concept of carrying capacity is especially relevant in the agricultural context. On a farm there is more concentration on species used to supply food, so less competition with other species, as biodiversity has been lowered. Predation is generally prevented also.

An arable farmer will know the average yield of crops from each field each year, as well as factors causing variation.

And a farmer keeping livestock will be concerned about the quantity and quality of plant growth in fields used by animals for pasture.

Environmental stewardship is a term used to refer to agricultural practices based on sustainability of resources within the environment, permitting land to be passed on to future generations without negative issues.

Estimating population size

The exact numerical value for the size of a population is not often known.

It is usually necessary to estimate the population size of a representative section within an ecosystem.

The method used for this depends on the lifestyle of the species involved.

A single Biologist (or team of them!) cannot cover the whole area under study, and certain animals only emerge at night.

The counting process itself may affect the distribution of the organism.

Quadrats and transects

For non-motile (sessile) organisms - principally plants - it is normal to use methods involving scoring distribution on sample areas within the wider part of the ecosystem.
This could conceivably also be used for slow-moving or small animals: snails?

Quadrats are (not necessarily square) areas - usually marked by a frame placed at several representative positions within the area of study. Often the area is marked with a pair of measuring tapes, at right angles. A random number generated by a computer or calculator, or looked up from a table in a book, can be used to give co-ordinates within the chosen area for the consistent placement of the quadrat - e.g. top left-hand corner always placed at this point. This is to avoid bias in choosing the sampling site.

Scoring can be on the basis of
abundance (counting the actual number of individual plants of the species within each quadrat)
frequency (index) - the number of times a plant species is present in a certain, large, number of quadrats, expressed as a percentage
percentage (ground) cover - usually assessed using a gridded quadrat. A 10x10 gridded quadrat has 100 'sub-squares' so counting the number of sub-squares containing (or at least half containing?) the target species gives a direct value for percentage cover. With a 5x5 grid, this would have to be done 4 times.

Quadrats are usually used in areas which are thought to be fairly uniform, but the same technique can be used to compare different areas, such as different fields or sections treated with different fertilisers or weedkillers.

Transects are samples - possibly taken using quadrats - but they are generally taken in a particular direction. This is when there is evidence that a particular environmental factor is affecting the distribution of the organisms involved, and its directionality is clearly defined, e.g. down a hill, towards a pond, or sideways away from a well-trodden path.

It is usual to take samples at predetermined intervals along a tape or string laid along the ground - a line transect.

If a series of quadrats are taken in a straight line with no gaps it is called a belt transect.
It is much more normal to move quadrats, leaving gaps - this is an interrupted belt transect.

Point quadrats (point frames) may be used at intervals along the line. These consist of a comb-like device and samples are taken at the tip of each point.

Other transects - parallel to the first - can act as replicates.

Don't forget:
A 0.5m square quadrat is 0.25m2 in area, so this needs to be borne in mind if converting your counts to values per m2, or scaling it up to give totals for the area in question.

Quadrats can be gridded or open.

10x10quadrat (66K) 5x5quadrat (209K) openquadrat (204K)

transect (63K)

Field survey work is usually a team activity

pointframe (59K)

This point quadrat has 10 pins which can be lowered to touch plants to be identified.

Mark-release-recapture method

This technique - for motile organisms: animals - has several stages - and must be carried out with due consideration for the organisms concerned, causing minimal distress.

Trapping (some of) the organisms using a suitable technique. Traps must be checked in good time.

Marking the trapped organisms using a method that does not affect their chances of survival.

Counting the number of organisms marked - N1

Releasing them back into the environment, as close as possible to the capture site. There must be an adequate time period for the released organisms to re-integrate with their environment. This will obviously vary according to the lifestyle of the organisms concerned, and the time of day!

Repeated trapping - surprisingly most animals do not learn to avoid getting caught twice by the same technique. Some organisms may actually seek out traps if they benefit from food left there before!

Counting the second sample - N2, and recording the number of previously marked individuals - NM.

Releasing the organisms again.

Calculating the estimated number of organisms in the sampling area. This can be done with the expression below:

N1 x N2

Trapping small mammals

Longworth_trap (28K) Plastic versions are also available
The Longworth trap is often used to catch various types of mice within a wood or field. It is generally baited with food items and a certain amount of vegetation as bedding material - animals may be in the trap for several hours before release. In the evening several of these traps are placed along the edges of spaces or near to animal runways or tracks. The animal trips the trap causing the door to shut as it passes to the end of the protruding section, and next morning it is easy to see which traps have been successful.
Some traps may have holes (of a definite, small size) to allow the escape of small active rodents that need to keep foraging all night, e.g. shrews.

rodent_in_bag_smaller (853K)
These traps are generally emptied into a polythene bag and the captured mammal can be examined. Marking is sometimes done by trimming a small amount of hair from the belly using scissors, or applying 'permanent' marker to the (whiter section of the) underbelly.

Catching invertebrates

trough_fun (72K)
snails in tray (28K)
The population of water snails in a water trough (tank used to store drinking water for animals) can be assessed by trawling them out of the water with nets, marking their shells with coloured nail varnish, then releasing them and repeating the catching/counting process the next day.

Arthropods - insects (beetles, ants, etc) and arachnids (spiders and mites) - can be caught in pitfall traps - generally based on plastic cups placed in holes in the ground made with a garden trowel. These need clippings of vegetation as food/shelter - but beware: predatory species may eat smaller specimens! It is normal to put a partial lid on the traps to ensure they do not fill up with rain. These specimens can be marked (on their undersides?) with nail varnish or white correcting fluid.

Assumptions made when using the mark-release-recapture method

No migration (into or out of the area) during the sampling time

No deaths or births during the sampling time

Random mixing within the population - and equal exposure to the trapping method - males may be out on the hunt (for food and mates) whereas females may stay 'at home'

Marks remain - they are not cleaned off/washed off

Marks do not affect viability of organisms - so they do not behave differently or become more vulnerable to predators


Ecosystems are dynamic in that they can change over time as various factors exert different influences on an area. The term succession covers the idea that certain species are successful because they have adaptations which allow them to colonise an area, and also the idea that one species succeeds or follows on from another.

Primary succession

This is a process of colonisation taking place in an area which lacks soil and vegetation, such as coastal sand dunes, land resulting from volcanic lava flow (including islands formed from submarine upheavals), or spaces left when glaciers melt and retreat.

It is normally seen in terms of competing plant species, although there is clearly a parallel development of animal species taking place alongside, or lagging behind, the different plant types.

Initially no living organisms are able to colonise the virgin surface which is unfavourable to plant growth: too hot or cold, too salty, and lacking suitable structure for root penetration. Over a period of time, various species may be carried by chance into the area - for example floating or windborne seeds or spores - but most are unsuccessful in the prevailing environmantal conditions.

Eventually, conditions change slightly, possibly as a result of 'weathering'. As a result, a single species, the pioneer species is able to withstand the inhospitable conditions and colonise (parts of) the area. It is usually a small organism and it has certain features which enable it to grow in the inhospitable environment. The ability to fix atmospheric nitrogen is quite important here. Pioneer species on rocky terrains are often algae, lichens or fungi but specialised higher plants such as Marram grass Ammophila spp may colonise and stabilise sand.

Conditions here will be somewhat changed as a result of the pioneer species growing: release of acidic secretions or root action may break rock into smaller particles and enlarge crevises, perhaps allowing better drainage. And decomposition of leaves dropped from living plants and the plant bodies of whole dead plants will add organic mater - humus - to the soil developing from the original rocky material. This will raise the water-holding capacity of this soil, as well as contributing mineral nutrients. As a result, the pioneer species may grow to occupy more of the exposed environment, but it may not be the whole area.

Other species - 'secondary colonisers' - may subsequently colonise this area and will cause more changes to the environment. Each stage is called a sere, with a characteristic seral community. These are likely to cause further changes to the soil or by growing taller they may give shelter from winds, but also provide shade from sunlight. They may thus change temperatures and prevent photosynthesis.

Aside from the gradual replacement of one plant species by another, noticeable changes in the speed of succession may result from the ingress of animals into the area. Nesting birds - especially seabirds visiting islands - will defecate and raise the fertility of soil without necessarily damaging plant colonisers by eating them, although this may occur if plant material is used in nesting material.

The changes may effectively make the environment less hospitable for the previous species and possibly more hospitable for subsequent colonisers. This is likely to result in an increase in biodiversity in the area.

The climax community is the final stage in the succession process. It represents a fairly stable community of plants, animals and fungi.

Surtsey - the island showing succession in living memory

surstey-1964-volcanic-eruption (47K)
The island of Surtsey - to the south of Iceland - was formed by submarine volcanic eruption lasting from November 1963 to June 1967. Although its area was initially 2.7km2, it has subsequently been eroded by sea and weather to 1.5km2. It is about 30% lava and 70% tuff (ash compacted into a soft solid rock after a process of consolidation), and these areas differ somewhat chemically and physically. Access to it is strictly controlled, to avoid damage to the developing ecosystem.

Sea sandwort
Honckenya peploides
sea_sandwort (321K) This succulent perennial herb has creeping stolons which help it to become established on shingle beaches , and it was the first recorded flowering plant on the island
Sea lyme grass
Leymus arenarius
220px-Leymus_arenarius_habitus (15K) This is a sand-loving, salt tolerant grass species and the main secondary coloniser at the shoreline

The plant and animal species which have colonised it derive from other Vestmannaeyjar islands to the north, which share a similar volcanic origin. It was originally expected to find lichens as first colonisers (pioneeer species) but mosses were the first to be seen - 18 species by 1970 - and 'vascular plants' followed soon after (see either side).

The island was visited by various seabirds and eventually adopted as a nesting site by Black guillemot Cepphus grylle in 1970, then fulmars Fulmarus glacialis, followed by a number of gull species from 1985 onwards.
Within the tephra sand and lava based areas of the gull colony, different plant species have become well established, whereas other parts of the island are less hospitable and remain only partly colonised.
surtsey_02 (254K)

The number of plant species on the island is now more than 70, and now includes a small shrub - the tea-leaved willow Salix phylicifolia. A number of land-based bird species have now also become established.

Sand dune succession

Beach grass, shrub-bunchgrass, conifer trees and hardwood trees are said to be examples of species (?) involved in succession over a considerable period of time.

marram grass dune (531K)
Ammophila arenaria on a sand dune

TS Ammophila arenaria leaf (curled)

marram grass leaf (56K) The stomata are in sunken pits and the hair-like projections act as baffles to maintain a humid environment.
Marram grass (Ammophila arenaria) is a type of beach grass that shows adapations which allow it to grow in sand at the seashore a few metres above high water mark. It has strong salt-tolerant rhizomes which spread out sideways and trap and stabilise sand blown in from the shoreline so that sand dunes are formed. Its strap-like leaves can roll up, reducing water loss in dry and windy conditions.

bunchgrassinspring (7K)

Bunchgrass also stabilises sand and covers more area so that changes can occur in the sand beneath.

Secondary succession

This is the series of changes which take place on a established ecosystem, i.e one which was previously colonized, but its community structure has been disturbed or damaged.

Examples include areas which have been deforested after tree-felling or subject to fires, woodland cleared to create farmland or farmland which has been 'left to Nature'.

Habitat conservation

It is often thought that areas of the environment need intervention by Man in order to maintain them for various reasons:
as a habitat for certain species considered worthwhile or 'vulnerable'
for agricultural purposes
for 'sporting' activities
for other human needs
to mitigate other (human) activities e.g. road building, urban creep.

In these cases, it is necessary to take action which affects succession and this may be seen to be a source of conflict between different parties, often with different motivations and philosophies.

Moorland is a type of habitat much liked by the public: walkers, bike riders and horse riders, and nature lovers in general. There are many areas managed for public amenity access. However large areas are also cultivated by landowners to support grouse for shooting - a controversial issue in its own right.

However moorland needs to be maintained to prevent development of trees such as silver birch which would shade the ground and reduce the coverage by heathers (ling, bell heather and cross-leafed heath). This has been carried out using controlled burning, grazing by cattle and especially sheep, or deliberate clearance of trees by volunteers. I note that heathland firing ranges near me are mowed using heavy machinery.

Lawn mowing is an example of a plant community being maintained by human activity so as to prevent succession and the progress to a more diverse ecosystem. Grass can survive being cut back - its growing points are at low level, whereas other plant species generally have buds at the tips of shoots which are removed and hence do not usually develop into taller specimens.

In some places, large areas are seen as natural but it not realised that the original dominant plant life has been removed and never allowed to re-establish itself. This is particularly true of farmland; grazing by cattle maintains the character of the fields, and grazing by sheep ensures that upland hillsides stay covered in grass.

Other related topics on this site

(also accessible from the drop-down menu above)

Similar level
Evolution processes - The basic stages of Evolution
Allopatric speciation - Evolution based on geographical isolation
Sympatric speciation - Evolution based on reproductive isolation, not geographical isolation
Species and taxonomy - Putting a name to to a species
Biodiversity - Biological diversity - the variety of life.

Alternatives to fieldwork

Virtual Ecology Tours by Gary Skinner (who would appreciate feedback ). There are two main sections: the rocky shore page and the sand dunes page

Web references

Arctic hare From Wikipedia, the free encyclopedia

Biology Fieldwork (A Level) from the FSC (Field Studies Council)
Vegetation sampling
Mark release recapture
Sand dunes

Celebrating 60 years and Counting of the Longworth Mammal Trap blog from the NHBS (originally Natural History Book Service, but now supplies more)

Heather is the dominant species of open heathland - New Forest National Park Authority

The volcano island: Surtsey, Iceand

Type catalogue of reptiles in the Zoologische Staatssammlung München

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